1. Field of the Invention
The present invention relates to an internal combustion engine control apparatus and more particularly to an internal combustion engine control apparatus provided with an exhaust gas recirculation amount estimation unit fort estimating an exhaust gas recirculation amount utilized in controlling an internal combustion engine.
2. Description of the Related Art
In order to appropriately control an internal combustion engine, it is important to accurately calculate the amount of air taken into a cylinder and to perform fuel control and ignition timing control in accordance with the amount of air taken into the cylinder flow. In general, as the methods of measuring the amount of air taken into a cylinder of an internal combustion engine, there exist two kinds of methods, i.e., a method (hereinafter, referred to as an AFS method) in which an air flow rate is measured by an air flow sensor (hereinafter, referred to as an AFS (Air Flow Sensor)) provided at the upstream side of the throttle valve in an intake pipe of an internal combustion engine and a method (referred to as a S/D (Speed Density) method) in which there is provided an intake manifold pressure sensor for measuring the inner pressure of an intake manifold (hereinafter, referred to as an intake manifold pressure) as the generic name of an intake pipe including a surge tank and the intake manifold, situated at the downstream side of the throttle valve in an intake pipe, and the flow rate of air taken into a cylinder is estimated based on the intake manifold pressure measured by the intake manifold pressure sensor and the rotation speed of the internal combustion engine. In addition, there exist also a method in which these sensors are concurrently provided and the foregoing methods are switched in accordance with the driving state and a method which is an AFS method but utilizes a measured intake manifold pressure.
With regard to fuel control in an internal combustion engine, when feedback control can be performed in such a way that, primarily, a fuel, the amount of which causes a desired air-fuel ratio for a cylinder intake air flow rate, is injected, almost excellent controllability can be obtained; however, ignition timing control needs to be performed at an ignition advance angle which causes an maximum output (referred to as an MBT (Minimum Spark Advance for Best Torque), hereinafter) in accordance with not only the rotation speed of the internal combustion engine and the cylinder intake air flow rate but also other factors such as the temperature of the internal combustion engine, whether or not a knock has occurred, the fuel property, and the exhaust gas recirculation ratio (referred to as an EGR (Exhaust Gas Recirculation) ratio, hereinafter). The EGR ratio denotes the ratio of the EGR amount to the intake air flow rate.
Among the foregoing factors that provide effects to the MBT, for example, the temperature of the internal combustion engine and whether or not a knock has occurred can be detected by a coolant temperature sensor in the internal combustion engine and a knock sensor, respectively; with regard to the fuel property, it can be determined whether the fuel is regular gasoline or high-octane gasoline, based on whether or not a knock has occurred.
Meanwhile, with regard to the EGR ratio, there are two methods, i.e., a method (referred to as an external EGR, hereinafter) in which an EGR valve is provided in an EGR path that connects the exhaust pipe with the intake pipe of the internal combustion engine and the EGR amount is controlled based on the opening degree of the EGR valve and a method (referred to as an internal EGR) in which a variable valve timing mechanism (referred to as a VVT (Variable Valve Timing), hereinafter), which makes the opening/closing timings of an intake valve and an exhaust valve variable, is provided and depending on the opening/closing timing of the VVT, an overlap period, during which the intake valve and the exhaust valve are concurrently opened, is changed so that the amount of EGR, which is caused by remaining exhaust gas in the cylinder, is controlled; in some cases, both the methods are concurrently utilized. In the external EGR control method, the EGR ratio can approximately be calculated from the opening degree of the EGR valve, the exhaust pressure, and the inner-intake-pipe pressure.
In the following explanation, an EGR and an EGR ratio, when simply expressed in this manner, denote an external EGR and an external EGR ratio, respectively.
Because in recent years, in order to further reduce the fuel cost and further raise the output, there is commonly utilized an internal combustion engine having an external EGR and a VVT for an intake valve and an exhaust valve (hereinafter, referred to as an intake/exhaust VVT), the amount of air taken into the cylinder from the intake manifold largely changes depending on the opening degree or the valve timing of the EGR valve; therefore, unless the effect of the opening degree or the valve timing of the EGR valve is considered, the accuracy of calculating the amount of air taken into the cylinder in the whole driving region including the steady and the transient mode is largely deteriorated, especially, in a S/D method. Because when the opening degree or the valve timing of the EGR valve is changed, the response is delayed, the fact that during transient driving, the changed opening degree or the valve timing of the EGR valve does not coincide with the opening degree or the valve timing of the EGR valve, which has been set during steady driving, causes the accuracy of calculating the air flow rate to largely deteriorate.
Therefore, to date, as a method, according to the S/D method, of estimating a cylinder intake air flow rate, the method disclosed in Patent Document 1 has been proposed. Patent Document 1 discloses that the amount of air taken into a cylinder is calculated from the intake manifold pressure MAP, the volume efficiency VE, the cylinder volume V, and the temperature T; however, in Patent Document 1, it is assumed that the parameters of an internal combustion engine such as the opening degree and the valve timing of the EGR valve do not change. Provided EGR or the intake/exhaust VVT is applied to the S/D method disclosed in Patent Document 1, it is conceivable that the volume efficiency VE at a time when the opening degree of the EGR valve coincides with the control map for the opening degree of the EGR valve or at a time when the valve timing coincides with the control map for the valve timing is set as a map value; however, although posing any problem during steady driving, the foregoing method causes the accuracy of calculating the air amount to largely deteriorate during transient driving. Thus, in accordance with the opening degree or the valve timing of the EGR valve, a great number of maps for the volume efficiency VE are prepared so that the accuracy of calculating the air amount can be suppressed from deteriorating during transient driving.
In recent years, it has become common that an internal combustion engine is controlled by utilizing, as an index, the output torque of the internal combustion engine; even when the output torque is estimated, the thermal efficiency changes in accordance with the cylinder intake air flow rate and the EGR ratio. Accordingly, in order to calculate the foregoing MBT and furthermore in order to estimate the torque and the thermal efficiency, it is required to accurately calculate the cylinder intake air flow rate and the EGR ratio. In order to accurately obtain the EGR ratio, it is required to accurately calculate the EGR flow rate.
Therefore, to date, as a method of calculating an EGR flow rate and an EGR ratio, the method disclosed in Patent Document 2 has been proposed. Patent Document 2 discloses a method in which the EGR flow rate is calculated based on an exhaust gas amount obtained from the opening area of an EGR valve and an exhaust gas amount obtained from an opening area command value for the EGR valve and then the EGR ratio is estimated. With a simple configuration, the method disclosed in Patent Document 2 make it possible to calculate an EGR flow rate, by utilizing a preliminarily provided “EGR-valve opening degree vs. flow rate characteristic” and the opening area of an EGR valve.